Use empty itemstack, not null in PlayerInteractManager

Introduced in 00a41bca42
Fixes GH-1642

Spigot-Server-Patches/0402-MC-136865-Use-valid-item-for-enchantment-checks-on-b.patch

@@ -1,4 +1,4 @@
-From a101ae9254d474be1b9817bb5970ca62414fcdcc Mon Sep 17 00:00:00 2001
+From 4b3504dfbcfdcfe361c11919ea7c6597f05b2d5d Mon Sep 17 00:00:00 2001
 From: MisterVector <whizkid3000@hotmail.com>
 Date: Thu, 1 Nov 2018 14:50:05 -0700
 Subject: [PATCH] MC-136865: Use valid item for enchantment checks on block
@@ -13,14 +13,14 @@ keep the clone of the item used to a non empty value so it represents
 the item used.
 
 diff --git a/src/main/java/net/minecraft/server/PlayerInteractManager.java b/src/main/java/net/minecraft/server/PlayerInteractManager.java
-index 23fc4d8e14..599bdf054f 100644
+index 23fc4d8e1..0b6c25936 100644
 --- a/src/main/java/net/minecraft/server/PlayerInteractManager.java
 +++ b/src/main/java/net/minecraft/server/PlayerInteractManager.java
 @@ -375,10 +375,11 @@ public class PlayerInteractManager {
                      ItemStack itemstack1 = this.player.getItemInMainHand();
                      boolean flag1 = this.player.hasBlock(iblockdata);
  
-+                    ItemStack itemstack2 = flag && flag1 && event.isDropItems() && !itemstack1.isEmpty() ? itemstack1.cloneItemStack() : null; // Paper - MC-136865 - clone before use
++                    ItemStack itemstack2 = flag && flag1 && event.isDropItems() && !itemstack1.isEmpty() ? itemstack1.cloneItemStack() : ItemStack.a; // Paper - MC-136865 - clone before use
                      itemstack1.a(this.world, iblockdata, blockposition, this.player);
                      // CraftBukkit start - Check if block should drop items
                      if (flag && flag1 && event.isDropItems()) {